The present invention relates to a composition and method for inhibiting the corrosion of metal parts by aqueous salt solutions present in the environment (e.g. aqueous NaCl solutions). The method includes coating said metal parts with a corrosion inhibitive liquid polymer composition and curing same to a solid elastomer. The non-toxic corrosion inhibitive polymer composition includes a liquid polysulfide, polyurethane, polythioether or polyether having incorporated therein a metal corrosion inhibitive effective amount of at least one ammonium salt of ortho- or hypophosphorous acid or phosphoric acid (ortho-, meta- or hypophosphoric acid).
The present invention is particularly well suited for inhibiting the corrosion of at least two metal parts which have a joint or space therebetween formed by the opposed mating surfaces of said metal parts which are secured together. In an attempt to prevent the corrosion of metal parts, the joint or space formed by the interface between the metal parts (particularly aluminum and/or aluminum alloy) is oftentimes filled with a liquid polymer cured to an elastomeric solid which helps to prevent aqueous salt solutions, as well as oxygen, from coming into contact with the mating surfaces of the parts which are joined. The problem of corrosion of aluminum containing metal parts (including aluminum alloys) is even worse in the case of aircraft and ships since the metal parts making up the aircraft and/or ship are oftentimes made of a number of metals including aluminum and/or aluminum alloys. When dissimilar metals are joined, corrosion is a serious problem when the space or joint formed by the interface between the mating surfaces of the parts is exposed to aqueous salt solutions. This is particularly true when aluminum (including aluminum alloys) is joined or secured to a dissimilar metal. For example, in the case of aircraft, aluminum and/or aluminum alloy panels are often secured together with cadmium, nickel, stainless steel or titanium rivets which cause severe corrosion problems when the spaces or joints between such rivets and panels are exposed to aqueous salt solutions particularly in the presence of oxygen. The same is equally true of ships which have aluminum superstructures joined to steel hulls.
In the past, exclusion of aqueous salt solutions, electrical insulation and sacrificial anodes between dissimilar metals have been the primary means employed to control corrosion of the metal parts. The large stresses and movements of the structures of both aircraft and ships have made the use of elastomeric sealants the preferred material to both exclude aqueous salt solutions and accommodate structural movements. In practice, however, many interfaces of metal structures sealed with elastomers become permanently contaminated with aqueous salt solutions which seriously attack and weaken structural components. In addressing this problem, U.S. Pat. No. 3,841,896 added to polysulfide elastomers water soluble alkaline earth chromates. Such chromates were very beneficial in producing corrosion inhibitive elastomeric polysulfides but the use of chromates has been severely restricted because of toxicity and disposal problems. Therefore it is desideratum in the art to produce corrosion inhibitive elastomeric polymers which are corrosion inhibitive but do not contain chromates or other toxic compounds.
Although, as noted, soluble chromates have been found beneficial, I have also found that they offer very limited protection in preventing aluminum alloy attack, particularly when coupled with dissimilar metals such as carbon fiber reinforced panels or titanium metal which is a common metal used as a fastener in the aircraft industry.